1. Field of the Invention
This invention relates generally to the deposition of particles on engine parts, and more particularly to a method and apparatus for reducing particle deposits on the surface of engine parts by coating the engine parts.
2. Description of the Related Art
Gas turbine engines use the well known Brayton cycle to generate a continuous flow of high-pressure, high temperature gas. In a typical gas turbine engine, air is drawn into a rotating compressor which compresses the air, and the air is then heated at constant pressure in a combustion chamber. The high-pressure, high temperature gas exiting the combustion chamber drives the turbine to produce rotational energy.
Gas turbine engines typically show decreasing compressor efficiency over time due to the accumulation of particles on the airfoil surfaces of the compressor. Accumulated particles can reduce the total airflow in the early compressor stages, reduce the pressure ratios in the later compressor stages, and initiate sulfide attack on the later compressor stages. Particles also build up on exhaust ducts as soot (carbon-based combustion by-products), particularly on the low flow and recirculation zone surfaces of the exhaust ducts.
In land-based turbines, filter houses have been utilized to reduce the number and size of particles entering the engine. The filter house comprises a series of particle traps which reduce the number of admitted particles, typically trapping substantially all particles which are larger than about 5 microns in diameter. However, such filter houses are not effective at trapping the smaller particles, e.g. less than about 5 microns in diameter, which pass through the filter into the engine. In addition, it is not generally feasible to use such filters on aircraft engines.
Soot production in gas turbine exhaust ducts is conventionally reduced through careful design of the combustor, i.e., by designing the combustor to efficiently bum substantially all of the fuel. However, it is difficult to produce a combustor which remains soot-free under all operating conditions.
Coatings are known which are applied to airfoil surfaces to protect the airfoils from corrosion. For example, Sermatech in Limerick, Pa. manufactures a coating (Sermatech 5380 DP) which includes aluminum particles and a phosphate glass seal. Such corrosion coatings, however, do not effectively reduce the susceptibility of the airfoil surface to particle deposits.
It would be desirable, therefore, to have a method and apparatus for reducing particle deposits on engine components which was effective and which could be applied to aircraft engines and other gas turbine engines to maintain compressor efficiency and reduce soot deposits on exhaust ducts.